The present invention relates to an electromechanical converting device used for an actuator in an industrial robot, an ultrasonic motor, etc., and in particular to an improvement of a laminate displacement device having a displacement increased by laminating a plurality of thin plates made of an electromechanical converting material through internal electrodes.
A laminate piezo-electric device used in a displacement device used for a positioning mechanism, a brake, etc. in an X-Y stage is fabricated by a method, by which an electrode is formed on each of thin plates made of a piezo-electric ceramic material formed in a predetermined shape to polarize it and thereafter they are stuck to each other directly or through thin metallic films with organic adhesive. However a displacement device laminated with adhesive as described above has drawbacks that the adhesive absorbs displacement due to vibration of the piezo-electric elements, depending on use conditions, that the adhesive is deteriorated by high temperature environment or long term use, etc.
For this reason, a laminate displacement device having a laminate chip condenser structure is used in practice. That is, as described e.g. in JP-B-59-32040, a paste-like piezo-electric ceramic material obtained by adding binder to raw material powder and kneading them together is formed in a thin plate having a predetermined thickness and a conductive material such as silver-palladium, etc. is applied on one or both the surfaces of this thin plate to form internal electrodes. A predetermined number of thin plates described above are superposed on each other, bonded with pressure and further formed in a predetermined shape. Thereafter, they are transformed into ceramic by sintering and external electrodes are formed on two side surfaces of a laminate body thus obtained. Since the laminate displacement device thus constructed is excellent in the close adhesion at the junction portion between the thin plates made of the piezoelectric ceramic material and the internal electrodes and at the same time thermal characteristics thereof are stable, it has advantages that it can be used satisfactorily in a high temperature environment, that deterioration is extremely small over a long period of time.
FIG. 3 shows an example of the structure of the laminate displacement device described above, named so-called alternate electrode type. In FIG. 3, reference numeral 1 represents a thin plate made of a piezo-electric ceramic material and a plurality of them are superposed on each other, putting alternately positive and negative internal electrodes 2a and 2b therebetween, to form a laminate body 5. The internal electrodes 2a and 2b are so formed that an insulating portion on one side of the thin plates protrudes outwards or it is exposed and connected with external electrodes 3a and 3b, respectively, extending in the lamination direction, which are in turn connected with leads 6 through solder 7.
By the construction described above, when DC voltage is applied from the external electrodes 3a and 3b, electric field is produced between the internal electrodes 2a and 2b and the thin plate 1 is extended in the thickness direction by the longitudinal effect of the piezo-electric ceramic material, which gives rise to displacement. However, in such a construction, since the electric field intensity is low at the peripheral portions close to the side surfaces, i.e. portions, where the internal electrodes 2a and 2b are not superposed on each other, not only no deformation takes place there but also they hinder deformation of the whole device. Consequently, in such an alternate electrode type device, it is not possible to obtain an amount of strain proper to the electromechanical converting material with a high precision and further stress concentration takes place at the boundary between the displacement portion and the non-displacement portion. Therefore it has a drawback that the device is destroyed by application of a high voltage or by application of a voltage over a long period of time.
As a device, for which the drawback described above is removed, there is known a laminate displacement device, as indicated in FIG. 4, which is designated so-called whole surface electrode type, for which the piezo-electric displacement effect is increased (refer to e.g. JP-A-58-196068, etc.). In FIG. 4, identical parts are represented by reference numerals identical to those used in FIG. 3. The internal electrodes 2a and 2b are formed so as to be extended over the whole surface of the thin plate 1 and a predetermined number of thin plates are superposed on each other similarly . to that described previously. Then, on one of the side surfaces of the laminate body 5 thus constructed, an insulating layer 4 made of an insulating material is disposed for every two layers, covering an edge of either ones of the internal electrodes 2a and 2b (e.g. only the internal electrodes 2b) and further the external electrode 3a made of a conductive material is disposed, covering the side surface including the insulating layer 4. On the other hand, on the other side surface of the laminate body 5, an insulating layer 4 is disposed on the edge of the internal electrodes (e.g. 2a), on which the insulating layer stated previously has not been disposed, similarly to that described above, and the other external electrode 3b is disposed on the other side surface including the insulating layer 4. The operation of the displacement device constructed as described above is identical to that described above, referring to FIG. 3. However, in a displacement device having such a construction, more uniform deformation can be obtained than that produced in the construction indicated in FIG. 3 and therefore no stress concentration takes place. Consequently a large amount of strain proper to that electromechanical material can be obtained and therefore it has an advantage that no destruction at deformation takes place.